To optimize current consumption in the receiver of a transponder equipped with a battery, a typical case is to use polling method. In such a polling method, the receiver is active (ON state) for a certain ON time period (or polling-on period) followed by an OFF time period (or polling-off period). This ON-OFF sequence is repeated continuously during the transponder's listening or standby mode. Alternatively, possible scenario with the receiver is to be constantly ON (OFF time period=0).
A typical telegram comprises a sequence with different parts defining:                a continuous wave (CW) and/or a Preamble which is used in particular for analog circuitry settling in the transponder's receiver;        a Sync pattern which is used by the transponder to synchronize the data decoder with the incoming data stream (ideally this is a unique pattern which never occurs in the wake-up pattern or in the data section);        a Wake-up pattern (Identification pattern) that the transponder can decode for detecting whether the telegram is addressed to this transponder or not (so the microcontroller will be woken up only if the telegram is addressed to this transponder);        a Data section; and        an ‘EOT’ section indicating the end of the telegram transmission.        
In the listening or standby mode, when starting polling-on period, a classical transponder is first checking each channel for any activity. For minimum power consumption such checking is done through a wake-up circuit which consumes less power than the whole receiver circuit of the transponder. The patent publication US 2010/0245153 discloses such a transponder. This document concerns an active transponder able to receive amplitude modulated interrogation signals. The transponder includes a high frequency input amplifier with a relatively broad bandwidth. Such an amplifier has relatively large electric power consumption and will also amplify interference signals received by the transponder with a modulation frequency closed to the predefined one for this transponder. In order to limit the power consumption in a standby mode, the document US 2010/0245153 proposes to arrange a passive envelope detector (8) between the antenna and the input amplifier; i.e. to arrange this input amplifier between two parts of the demodulator respectively formed by the envelope detector and a kind of analog-to-digital converter named ‘circuit for demodulating’ in this document. Then, in the standby mode, an activation unit defining a wake-up circuit is provided for controlling the activation of the input amplifier and the circuit for demodulating. This activation unit receives as input signal an envelope generated by the envelope detector and it detects through a low frequency circuit whether the modulation frequency of an interrogation signal received by the antenna corresponds to the predefined one. If this is the case, only then the high frequency input amplifier and the analog-to-digital converter are activated to demodulate the interrogation signal.
The active transponder described in US 2010/0245153 is interesting for resolving the power consumption problem of high frequency input amplifier. However, there are other problems which remains, in particular a sensitivity problem for the transponder. There are two reasons for this sensitivity problem. First the envelope detector is placed before the input amplifier so that this amplifier only amplifies the envelope of the received interrogation signal which may already have some distortion due to the passive envelope detector. Secondly, this passive envelope detector itself has a low sensitivity. Due to the specific design of the disclosed transponder with an activation unit which needs as input an envelope signal and due to the fact that no active element can be implemented before this activation unit in order to avoid, according to the aim of US 2010/0245153, high power consumption in the standby mode, the transponder described in this document has a relatively low sensitivity.